GNU LGPL 2.1 (LGPL-2.1)
Library copyleft. Dynamic linking lets proprietary code use it freely. Static linking requires you to enable user relinking (provide object files or use dynamic). Modifications to the library itself must be published under LGPL. Can be upgraded to GPL.
Key facts
- SPDX id
- LGPL-2.1-only
- Category
- Weak copyleft
- Copyleft scope
- linking
- Express patent grant
- No
- SaaS triggers disclosure
- No
- GPL-compatible
- Yes — compatible with the GPL.
Obligations
When redistributing as open source.
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
- LGPL-2.1: publish the source of any modifications to the library itself.
- LGPL-2.1: modifications to the library must remain under LGPL-2.1.
Internal use only (never distributed)
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
Public SaaS / hosted web service
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
Static linking
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
- LGPL-2.1: publish the source of any modifications to the library itself.
- LGPL-2.1: modifications to the library must remain under LGPL-2.1.
- LGPL-2.1: provide object files or another mechanism so users can relink against a modified version of the library.
Commercial use
Yes. Copyleft is scoped to the library — your own application code can stay proprietary. Publish modifications to the library itself. Static linking requires a relink path for users.
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
- LGPL-2.1: publish the source of any modifications to the library itself.
- LGPL-2.1: modifications to the library must remain under LGPL-2.1.
Permissive: include notices (Apache also needs NOTICE + change statements). Weak copyleft: disclose only the modified library/files and, for LGPL static linking, provide a relink path. Strong copyleft (GPL): NOT possible to keep proprietary if GPL code is in the derivative work. AGPL: same as GPL plus network rules.
FAQ
- Can I use LGPL-2.1 in a commercial product?
- Yes. Copyleft is scoped to the library — your own application code can stay proprietary. Publish modifications to the library itself. Static linking requires a relink path for users.
- Do I have to open-source my code if I use LGPL-2.1?
- Only modifications to the library itself. Code that links against the library is not affected, subject to the linking terms.
- Does running LGPL-2.1 software as a SaaS require source disclosure?
- No. Hosting is not distribution under LGPL-2.1; running it as a service does not trigger source disclosure.
- Is LGPL-2.1 compatible with the GPL?
- Yes — compatible with the GPL.
Compatibility with other licenses
- Is LGPL-2.1 compatible with MIT? Yes
- Is BSD-2-Clause compatible with LGPL-2.1? Yes
- Is BSD-3-Clause compatible with LGPL-2.1? Yes
- Is ISC compatible with LGPL-2.1? Yes
- Is Apache-2.0 compatible with LGPL-2.1? Yes
- Is BSL-1.0 compatible with LGPL-2.1? Yes
- Is LGPL-2.1 compatible with Unlicense? Yes
- Is CC0-1.0 compatible with LGPL-2.1? Yes
- Is LGPL-2.1 compatible with MPL-2.0? It depends
- Is EPL-2.0 compatible with LGPL-2.1? It depends
- Is LGPL-2.1 compatible with LGPL-3.0? It depends
- Is GPL-2.0-only compatible with LGPL-2.1? Yes
- Is GPL-2.0-or-later compatible with LGPL-2.1? It depends
- Is GPL-3.0-only compatible with LGPL-2.1? It depends
- Is AGPL-3.0 compatible with LGPL-2.1? It depends